1. Field of the Invention
This invention relates to measurement of rotary position.
2. Prior Art
Various methods for linear or rotary position sensing are known in various applications. For example, capacitive and inductive position sensors are known.
In one particular application, the movement of a throttle valve in a motor vehicle engine is responsive to the demand of the motor vehicle operator. Preferably, the air/fuel mixture is controlled in response to the throttle position, and numerous sensors for generating a control signal in response to the throttle position have been developed. However, many previously known sensors are contact sensors in which the mechanical movement of the throttle is traced by mechanical displacement of electrical conductors in order to electrically represent the change in throttle positions. Such sensors are subject to mechanical wear that can substantially affect the efficiency and longevity of the electrical conductors and other moving components. Moreover, inherent electrical noise in the output signal limits resolution to a relatively large increment of angular displacement.
For noncontacting magnetic rotary position sensors the magnetic field sensing material is typically either a permalloy magnetoresistor or a Hall effect sensor. The rotary position sensors which use the Hall effect sensor measure the magnitude of the field along a fixed direction. Using rotating permanent magnets and/or flux dividers, this field magnitude is made to vary with rotation angle. Though these sensors can provide a linear output with rotation angle, in typical configurations they are affected directly by variations in permanent magnet strength and by variations in the sensing material, especially with temperature. In its final package, the sensor output is typically calibrated for these effects which increases the cost. The rotary position sensors which use permalloy magnetoresistors can be configured to measure magnetic field direction rather than magnitude. However, in commercially available devices, they provide a linear output over a limited angular range (typically 30.degree.). Throttle angular position measurement must span 90.degree..
An alternative magnetic sensor for use in angular position monitoring is the large magnetoresistive observed certain metal multilayer films. This effect, referred to as giant magnetoresistance (GMR), was first reported for Fe/Cr multilayer films, and later for Co/Cu multilayer films. FIG. 1 shows the decrease in resistance of a Co/Cu multilayer film for the cases of the field applied in the plane of the film as well as perpendicular to it. With field applied in the plane of a Co/Cu multilayer film, the resistance decreases from a maximum value of R.sub.max at zero field and saturates to a minimum value of R.sub.sat at .+-.H.sub.sat. A much larger field is required to saturate the resistance when it is applied perpendicular to the film plane. FIG. 2 shows the dependence of the resistance on the angle .theta., of a fixed magnetic field, H.sub.o, relative to the film plane. Angle .theta. is the angle between the plane of any element (i.e., the plane being defined as the surface tangent of the element) and the magnetic field, such that when the field is applied perpendicularly to the plane of the film, such angle equals 90 degrees. Here, the field magnitude is fixed at H.sub.o =1.5H.sub.sat. When the field is applied perpendicular to the plane of the film, .theta.=90.degree., the resistance is at a maximum value (.apprxeq.R.sub.max).degree.. The resistance saturates to a minimum value at .theta.=(90.+-..theta..sub.max).degree.. The width of the R vs .theta. curve is dependent on the ratio H.sub.sat /H.sub.o.
Like the smaller magnetoresistance seen in permalloy, GMR has a practical disadvantage of being temperature dependent. The variation with temperature of R.sub.max, R.sub.min, and H.sub.sat may require additional calibrations in certain applications. In addition, the resistance change of a typical GMR sensor is linear with field or angle only in limited ranges. To overcome these limitations, this invention uses a normalizing process that provides a linear output over a 90.degree. angular range which is unaffected by small variations in these parameters. Also, a multilayer material and structure are chosen which minimizes the effects of magnetoresistive hysteresis which can be a disadvantage in certain practical applications.